(1) Field of the Invention
The present invention relates to the field of rotary wing aircraft having at least one rotor driven in rotation by a power plant. The present invention relates more particularly to devices for synchronizing the movement of hydraulic members forming parts of the rotorcraft rotors for the purpose of varying the aerodynamic angle of incidence of the airfoils of the blades of such a rotor.
(2) Description of Related Art
Rotorcraft are fitted with at least one main rotor of substantially vertical axis providing the rotorcraft at least with lift. In the specific circumstance of a helicopter, the main rotor also provides the rotorcraft with propulsion and guidance in pitching and in roll.
Rotorcraft are also commonly fitted with at least one anti-torque device such as an auxiliary rotor having a substantially horizontal axis and suitable for providing the rotorcraft with stability and guidance in yaw. Such an auxiliary rotor may for example be a tail rotor, or it may equally well be at least one propulsive propeller in a high-speed helicopter having propellers for forward propulsion.
In order to vary the flight attitude of a rotorcraft, the blades making up the rotary wing of said at least one rotor are mounted so as to be individually hinged to a hub of the rotor so as to pivot about a pitch variation axis that extends substantially along the general extension direction of each blade. A pilot issues flight commands that cause the blades to pivot about their respective pitch variation axes cyclically and/or collectively, thereby causing variation in the aerodynamic angle of incidence of the airfoil of each blade. The pilot may equally well be a human pilot or an autopilot.
The blades of a rotorcraft rotor can be driven to pivot about their pitch variation axes by means of a drive member to which the blades are individually connected via respective pitch variation links.
For example, for a main rotor where it is desired to be able to vary the pitch of the blades cyclically and/or collectively, the member for varying the pitch of the blades is commonly arranged as a pair of plates, commonly referred to as swashplates.
The pair of plates are mounted to move axially along a mast that carries the main rotor and also to tilt relative thereto about a ball joint. Moving the pair of plates axially along the axis in which the rotary wing of the main rotor is driven in rotation causes the pitch of the blades to be varied collectively. Titling the pair of plates about the ball joint causes the pitch of the blades to vary cyclically.
Also by way of example, for an auxiliary rotor, the blade drive member is commonly arranged as an arm or as a plate depending on the number of blades in the auxiliary rotor. The member for varying the pitch of the blades is then mounted to move at least axially along the rotary drive axis of the auxiliary rotary wing in order to cause the pitch of the blades to vary collectively under the effect of the pitch-varying member moving axially.
In this context, the member for varying the pitch of the blades of a rotorcraft rotor about their pitch variation axes is commonly actuated by servo-controls.
Such servo-controls serve to provide a human pilot with assistance concerning the forces that the pilot needs to deliver in order to vary the pitch of blades by acting on a mechanical linkage for transmitting flight control movements resulting from a human pilot operating a flight control member.
Servo-controls may also be controlled by an actuator as a result of flight commands issued by an autopilot or as a function of electrical flight commands issued by a flight control member that is in turn operated by a human pilot.
In this context, aviation leads to a problem of ensuring an aircraft is safe, even in the event of a failure of servo-controls used to drive a member for varying the attitude of the aircraft under the effect of flight commands issued by a pilot. Such an attitude-varying member of an aircraft is constituted in particular by the blades of a rotorcraft as described above or indeed also conventionally by a flight control surface in an airplane.
That is why it is common practice to drive a single attitude-varying member of an aircraft by means of a plurality of servo-controls, each having a single double-acting actuator cylinder or by means of a servo-control having multiple double-acting actuator cylinders. The various servo-controls each having a single actuator cylinder or the various actuator cylinders of a servo-control having multiple actuator cylinders are fed individually with fluid using respective hydraulic circuits from fluid sources that are allocated thereto respectively. These arrangements are such that in the event of a malfunction of one of the hydraulic circuits, at least one other hydraulic circuit can be used on its own to drive the attitude-varying member of the aircraft.
Conventionally, single actuator cylinder servo-controls or the actuator cylinders of a multiple actuator cylinder servo-control are fed individually with fluid from fluid sources that are respectively allocated thereto via hydraulic directional-control valves that are actuated jointly in compliance with flight commands issued by the pilot.
Depending on the flight commands given by the pilot, the hydraulic directional-control valves share the flow of fluids between the various fluid sources and the various chambers of the actuator cylinders. Such hydraulic directional-control valves are frequently incorporated within servo-controls; however they may also be mounted independently on the ducts of the hydraulic circuits connecting the servo-control(s) to the various fluid sources.
Given the small cantilever extension of the member for pivoting blades about their pitch variation axes, it is conventionally preferred in a rotorcraft to make use of a servo-control having a hydraulic actuator with multiple hydraulic cylinders in order to actuate said pitch-varying member.
For information about a technological background close to the present invention, reference may be made for example to Document FR 2 916 492, which describes a servo-control having multiple actuator cylinders and suitable for varying the pitch of blades in a rotorcraft rotor.
There is also a constraint of ensuring that pitch variation is accurately synchronized with the relative movements between the various actuator cylinders and the hydraulic actuator rod that is common to the various actuator cylinders. Such a constraint arises in particular because of the need to maintain complete separation between the hydraulic circuits that are independently feeding each of the actuator cylinders of the servo-control having multiple actuator cylinders.
Document U.S. Pat. No. 3,543,641 or FR 1 529 793 describes an electrohydraulic device for controlling aerodynamic members of airplanes or aerodynes. For each of the controlled members, that device comprises: a hydraulic motor unit having two actuators powered by independent hydraulic circuits, two mechanically coupled-together directional-control valves associated with respective ones of the actuators in order to control the feed of hydraulic fluid thereto, and two receivers of electrical control pulses acting on the directional-control valves. Finally, two switch members associated respectively with the receivers set up or interrupt their connections with the directional-control valves. The two driving hydraulic actuators associated with each of the control members are arranged in tandem. The device has two identical electrical systems for control and servo-control. Each of those systems includes a corresponding receiver and switch on each motor. Switch members are provided so that in the event of one of the control systems operating poorly in service, the other system can automatically take its place. Other switch members are also provided so that in the event of a failure in the second system, the switches of each motor put the associated receivers simultaneously into action at the same time as the receivers are actuated electrically to place the directional-control valves of the two actuators in a position for returning the controlled aerodynamic members to a safe position.
In the context of an airplane, proposals have been made to synchronize the operations of a plurality of servo-controls having single-actuator cylinders that jointly move a control surface by balancing the pressures of the fluids flowing respectively in the end chambers and in the head chambers of the various actuator cylinders.
On this topic, reference may be made for example to Documents U.S. Pat. No. 4,409,884 and U.S. Pat. No. 4,825,748. More particularly according to Document U.S. Pat. No. 4,825,748, a pressure-balance valve balances the respective pressures of the fluids flowing through two servo-controls having single-actuator cylinders that jointly move a control surface. Nevertheless, the organization of the pressure-balance balance valve described in Document U.S. Pat. No. 4,825,748 does not enable the respective hydraulic fluid feed circuits to be rigorously segregated between the two servo-controls.
Other documents have been considered in the context of the invention, namely: JPH04357311, U.S. Pat. No. 4,128,047, U.S. Pat. No. 6,178,867, and US 2005/132877.
Furthermore, still with respect to making rotorcraft safe, the operation of hydraulic members making up hydraulic installations dedicated to varying the pitch of blades needs to be checked regularly. Furthermore, it is desirable to warn a rotorcraft pilot in flight of a malfunction of such hydraulic installations that might be a source of danger.
That is why, in order to limit the frequency with which said hydraulic installations are subjected to checking and maintenance operations, and in order to provide the pilot with information in the event of malfunction, there is a constant search for ways of integrating devices that detect malfunction in said hydraulic members.
More particularly, specifically for servo-controls, malfunction is potentially due to a dormant failure in a hydraulic member controlling their operation, such as in at least one of said hydraulic directional-control valves, for example. It should be recalled that a failure is commonly said to be “dormant” in the event of said failure not being detected. Consequently, it would appear to be appropriate to detect such dormant failures better in order to preserve the servo-control and avoid it malfunctioning, or indeed to anticipate dealing with such a potential malfunction.
Nevertheless, systems for detecting how a hydraulic member is functioning are bulky, and incorporating them in said hydraulic member is often difficult to achieve. It should be observed that the servo-controls for varying the pitch of the blades of a rotorcraft rotor are placed in a hostile environment that is already crowded with equipment, thereby making it even more difficult to install a device for detecting a failure in the operation of such servo-controls.
As a result, in practice, dormant failures in the set of hydraulic members located in the fluid feed circuits of servo-controls are often difficult to detect, and instead regular checks are performed on the operation of servo-controls. On this topic, reference may be made by way of example to above-mentioned Document FR 2 916 492, which discloses a system for detecting a malfunction of the servo-control described in that document.
In the event of a malfunction of a servo-control being detected, a search is made for the cause of such a malfunction by acting during a maintenance operation to verify all of the hydraulic members in the various hydraulic circuits serving to feed fluids respectively to the various actuator cylinders of the servo-control.